Acidic local environments inhibit CO2 reduction catalysis on coinage metals

The carbon-efficient electrochemical reduction of CO 2 (CO 2 RR) requires acidic electrolytes to mitigate irreversible CO 2 capture via non-faradaic CO 2 uptake and (bi)carbonate formation. However, CO 2 RR only takes place in acidic electrolytes upon addition of spectator cations and the mechanistic basis for this requirement remains unclear. Herein, we present scanning voltammetry experiments conducted in bulk acidic electrolytes in conjunction with differential electrochemical mass spectrometry (DEMS) analysis of CO 2 consumption and product formation. By quantifying both faradaic CO 2 consumption via CO 2 RR, and non-faradaic CO 2 consumption via the formation of (bi)carbonate in real-time, we deconvolute the direct role of the added electrolyte cations from their indirect role in fostering a transient local pH swing. We find that across coinage metals and electrolyte compositions, CO 2 RR commences only upon substantial depletion of hydronium and coincides with non-faradaic CO 2 uptake. Doping Ag with Pt to promote hydronium consumption via efficient H 2 evolution serves to lower the overpotential for CO 2 RR onset in scanning voltammetry experiments by ~1 V. These findings together indicate that an interfacial pH swing is a pre-requisite for, rather than merely a consequence of CO 2 RR, and that the primary role of the supporting electrolyte cations is to establish a non-equilibrium interfacial ionic environment depleted in hydronium ions. To rationalize these findings, we propose that acidic pH environments foster the formation of *COOH, rather than *COO − , and that this protonated intermediate undergoes unproductive fragmentation to CO 2 and H*, contributing to H 2 evolution, but not CO 2 RR. This work highlights the value of pre-steady state real-time analysis of both faradaic and non-faradaic interfacial reactions and establishes a self-consistent mechanistic picture for CO 2 RR in bulk acidic electrolyte.